Method for producing polyvinylacetals

ABSTRACT

Polyvinyl acetals are obtained in a cost-effective and environmentally advantageous manner by employing as at least a portion of the aqueous acetalization medium, the mother liquor from a previous acetalization reaction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for preparing polyvinyl acetals,where the mother liquor from a preceding batch is used in the form ofthe aqueous medium in which the acetalization takes place.

2. Description of the Related Art

The preparation of polyvinyl acetals, which are obtained from thecorresponding polyvinyl alcohols by polymer-analogous reaction with theappropriate aldehydes, was disclosed as early as 1924, and since then awide variety of aldehydes have been used for preparing the correspondingpolyvinyl acetals. Polyvinyl acetals are prepared in a 3-stage process(polyvinyl acetate→polyvinyl alcohol→polyvinyl acetal), and theresultant products contain vinyl alcohol units and vinyl acetate unitsas well as vinyl acetal groups. Polyvinyl formal, polyvinyl acetacetal,and polyvinyl butyral (PVB) have achieved particular commercialimportance. The term “modified polyvinyl acetals” hereinafter meanspolyvinyl acetals which contain other monomer units as well as the threepreviously mentioned units vinyl acetate, vinyl alcohol, and vinylacetal.

The largest application sector for polyvinyl acetals is the productionof safety glass in automotive construction and for architectural uses,plasticized polyvinyl butyral films being used as an intermediate layerin glazing units. Another application sector for polyvinyl butyrals isuse in corrosion-protection coatings, found in EP-A 1055686, forexample. Polyvinyl butyrals are also used as binders in coatings, andspecifically in printing inks, one of the reasons here being their goodpigment binding power. Examples of these are the modified polyvinylbutyrals with low solution viscosity from DE-A 19641064, obtained viaacetalization of a copolymer having vinyl alcohol units and 1-alkylvinylalcohol units.

In the abovementioned three-stage process, polyvinyl acetals areprepared by preparing a solution of polyvinyl alcohol in water, treatingthis with acid, and precipitating the polyvinyl acetal from thissolution (mother liquor) via addition of aldehydes. The product isseparated from the mother liquor, where appropriate subjected to afurther treatment, washed, and dried. To increase the degree ofacetalization, the process of EP-B 513857 proposes carrying out theacetalization with use of a temperature gradient, i.e. increasing thetemperature during the precipitation.

SUMMARY OF THE INVENTION

An object of the present invention was to provide a less expensive andmore environmentally compatible process for preparing polyvinyl acetals.Surprisingly, it has been found that the quality of the mother liquor isso good, and remains so good even on repeated reuse, that it can bereused for preparing further polyvinyl acetals.

Surprisingly, it has been found that the quality of the mother liquor isso good, and remains so good even on repeated reuse, that it can bereused for preparing further polyvinyl acetals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a process for preparing polyvinyl acetalsobtainable by means of hydrolysis of a vinyl ester polymer and thenacetalization of the resultant polyvinyl alcohol with one or morealdehydes from the group consisting of aliphatic and aromatic aldehydeshaving 1 to 15 carbon atoms, where

-   a) the polyvinyl alcohol forms an initial charge in an aqueous    medium,-   b) where appropriate, acid is added to adjust to a pH of <1,-   c) the acetalization is initiated via addition of the aldehyde    component, and-   d) the precipitating polyvinyl acetal is separated via filtration    from the filtrate,-   which comprises using, at 10 to 100% by weight of the aqueous    medium, the filtrate of a preceding batch, and, where appropriate,    in a further batch, reusing the filtrate produced in d).

Suitable vinyl ester polymers are those based on one or more vinylesters of unbranched or branched carboxylic acids having 1 to 15 carbonatoms. Preferred vinyl esters are vinyl acetate, vinyl propionate, vinylbutyrate, vinyl 2-ethylhiexanoate, vinyl laurate, 1-methylvinyl acetate,vinyl pivalate, and vinyl esters of α-branched monocarboxylic acidshaving 5 to 11 carbon atoms, for example, VeoVa9^(R) or VeoVa10^(R)(trade names of Resolution Performance Products). Vinyl acetate isparticularly preferred.

Where appropriate, the vinyl ester polymers may also contain monomerunits which derive from one or more monomers from the group consistingof unsaturated mono- or dicarboxylic acids and their esters of alcoholshaving 1 to 15 carbon atoms, olefins, dienes, vinylaromatics, and vinylhalides. Suitable monomers from the group of the unsaturated mono- ordicarboxylic acids are acrylic acid, methacrylic acid, fumaric acid, andmaleic acid. Suitable monomers from the group of the esters of theunsaturated mono- or dicarboxylic acids are esters of unbranched orbranched alcohols having 1 to 15 carbon atoms. Preference is given tomethacrylic or acrylic esters, for example methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate,propyl methacrylate, n-, iso-, and tert-butyl acrylate, n-, iso-, andtert-butyl methacrylate, 2-ethylhexyl acrylate, norbornyl acrylate.Particular preference is given to methyl acrylate, methyl methacrylate,n-, iso-, and tert-butyl acrylate, 2-ethylhexyl acrylate, and norbornylacrylate. Suitable dienes are 1,3-butadiene and isoprene. Examples ofpolymerizable olefins are ethene and propene. Vinylaromatics which maybe copolymerized are styrene and vinyltoluene. From the group of thevinyl halides those usually used are vinyl chloride, vinylidenechloride, or vinyl fluoride, preferably vinyl chloride. The proportionof these comonomers is preferably selected in such a way that theproportion of vinyl ester monomer is >50 mol % in the vinyl estercopolymer.

Where appropriate, the comonomers may also contain other comonomers in aproportion which is preferably 0.2 -to 20% by weight, based on the totalweight of the comonomers. Examples are ethylenically unsaturatedcarboxamides and ethylenically unsaturated carbonitriles, preferablyN-vinylformamide, acrylamide, and acrylonitrile; ethylenicallyunsaturated sulfonic acids or their salts, preferably vinylsulfonicacid, and 2-acrylamido-2-methylpropanesulfonic acid. Other examples arepre-crosslinking comonomers, such as polyethylenically unsaturatedcomonomers, e.g. divinyl adipate, diallyl maleate, allyl methacrylate,butanediol diacrylate, or triallyl cyanurate, or post-crosslinkingcomonomers, such as acrylamidoglycolic acid (AGA), methylmethylacrylamidoglycolate (MAGME), N-methylolacrylamide (NMA),N-methylolmethacrylamide, allyl N-methylolcarbamate, alkyl ethers, suchas the isobutoxyether, or esters of N-methylolacrylamide, ofN-methylolmethacrylamide, or of allyl N-methylolcarbamate.

Particular preference is given to polyvinyl acetate, vinylacetate-1-methylvinyl acetate copolymers, and vinylacetate-(meth)acrylic acid copolymers.

The polymerization to prepare suitable polyvinyl esters takes place in aknown manner, preferably via bulk polymerization, suspensionpolymerization, or polymerization in organic solvents, particularlypreferably in alcoholic solution. Examples of suitable solvents andregulators are methanol, ethanol, propanol, isopropanol, andacetaldehyde. The polymerization is carried out at reflux at atemperature of 55 to 100° C., and is initiated by a free-radical routevia addition of familiar initiators. Examples of familiar initiators arepercarbonates, such as cyclohexyl peroxydicarbonate, or peresters, suchas tert-butyl perneodecanoate or tert-butyl perpivalate. The molecularweight may be adjusted in a known manner via addition of regulators, viathe solvent content, via variation of the initiator concentration, orvia variation of the temperature. Once the polymerization has concluded,the solvent is distilled off, as are, where appropriate, excess monomerand the regulator.

The hydrolysis of the vinyl ester polymers to give polyvinyl alcoholstakes place in a manner known per se, e.g. by the belt process orkneader process, in an alkaline or acidic medium, with addition of acidor base. The solid resin is preferably taken up in alcohol, such asmethanol, adjusting to a solids content of 15 to 70% by weight. Thehydrolysis is preferably carried out in a basic medium, for example viaaddition of NaOH, KOH, or NaOCH₃. The amount generally used of base is 1to 5 mol % per mole of ester units. The hydrolysis is carried out attemperatures of 30 to 70° C. Once the hydrolysis has concluded, thesolvent is distilled off, and the polyvinyl alcohol is obtained aspowder. However, the polyvinyl alcohol may also be obtained in the formof an aqueous solution via progressive addition of water while thesolvent is distilled off.

The partially or completely hydrolyzed vinyl ester polymers preferablyhave a degree of hydrolysis of 50 to 100 mol %, particularly preferably70 to 99 mol %, most preferably ≧96 mol %. Polymers described here ascompletely hydrolyzed are those whose degree of hydrolysis is ≧96 mol %.Partially hydrolyzed polyvinyl alcohols are those whose degree ofhydrolysis is ≧50 mol % and <96 mol %.

For the acetalization, the partially or fully hydrolyzed polyvinylacetates are taken up in an aqueous medium, the aqueous medium usedcomprising 10 to 100% by weight of the filtrate of a preceding batch.The solids content of the aqueous solution is usually adjusted to 5-30%by weight. The polyvinyl alcohol preferably forms an initial charge inthe form of an aqueous solution with a concentration of 10 to 50% byweight, and the desired solids content is established via addition ofthe filtrate (mother liquor) which derives from a preceding batch andcomprises water, acid, and a very small concentration of residualaldehydes. The mother liquor may be subjected to prior filtration inorder to minimize the solids content of preceding product remainingtherein. Where appropriate, the solids content for the precipitation mayalso be further adjusted to the desired value via addition of water.

The acetalization takes place in the presence of acid (acidiccatalysts), for example hydrochloric acid, sulfuric acid, nitric acid,or phosphoric acid, at a pH <1. If the pH obtained using the motherliquor is still too high, further acid is added. The pH of the solutionis preferably adjusted to a value <1 via addition of 20% strengthhydrochloric acid.

After addition of the acid, the solution is cooled, preferably to −10 to+30° C. The rule here is: the lower the molecular weight of thepolyvinyl alcohol used, the lower the selected precipitationtemperature. Once the precipitation temperature has been established,the precipitation of the polyvinyl acetal is initiated via addition ofthe aldehyde component.

Preferred aldehydes from the group of the aliphatic aldehydes having 1to 15 carbon atoms are formaldehyde, acetaldehyde, propionaldehyde, andmost preferably butyraldehyde, or a mixture of butyraldehyde andacetaldehyde. Examples of aromatic aldehydes which may be used arebenzaldehyde or its derivatives. The amount of aldehyde added heredepends on the desired degree of acetalization. Because theacetalization proceeds with almost complete conversion, the amount to beadded can be determined via simple stoichiometric calculation. Once theaddition of the aldehyde has concluded, the acetalization is completedvia heating of the batch to 20-60° C. and stirring for some hours,preferably 1 to 6 hours, and the precipitating polyvinyl acetal isisolated via filtration from the filtrate. The pulverulent reactionproduct is isolated after a washing step and drying step.

The mother liquor (filtrate) from which the batch has been precipitatedis preferably stored and used for a subsequent precipitation. Tostabilize the product, alkalis may also be added. During precipitationand further treatment, use may be made of emulsifiers in order tostabilize the aqueous suspension of the polyvinyl acetal.

In order to stabilize the aqueous suspension of the polyvinyl acetal,use may be made of anionic, zwitterionic, cationic, or non-ionicemuslifiers, or else protective colloids. It is preferable to usezwitterionic or anionic emulsifiers, where appropriate also in mixtures.Preferred non-ionic emulsifiers used are condensation products ofethylene oxide (EO) or propylene oxide (PO) with linear or branchedalcohols having 8 to 18 carbon atoms, or with alkylphenols, or withlinear or branched carboxylic acids having 8 to 18 carbon atoms, or elseblock copolymers of ethylene oxide and propylene oxide. Examples ofsuitable anionic emulsifiers are alkyl sulfates, alkylsulfonates, alkylaryl sulfates, and sulfates or phosphates of condensation products ofethylene oxide with linear or branched alkyl alcohols with 2 to 25 EOunits or with alkylphenols, and mono- or diesters of sulfosuccinic acid.Examples of suitable zwitterionic emulsifiers are alkyldimethylamineoxides, the alkyl chain having 6 to 16 carbon atoms. Examples ofcationic emulsifiers which may be used are tetraalkylammonium halides,such as C₆-C₆-alkyltrimethylammonium bromide. It is also possible to usetrialkylamines having one relatively long (≧5 carbon atoms) and tworelatively short hydrocarbon radicals (<5 carbon atoms), these beingpresent in protonated form during the course of the acetalization, whichproceeds under strongly acidic conditions, and being capable of actingas emulsifier. The amount of emulsifier is 0.01 to 20% by weight, basedon the total weight of the polyvinyl acetal in the mother liquor.Preference is given to amounts of 0.01 to 2% by weight of emulsifier,and a particularly preferred amount of emulsifier is 0.01 to 1% byweight, based on the polyvinyl acetal.

The polyvinyl acetals obtained by the inventive process have a degree ofacetalization of 1 to 80 mol %, preferably 45 -to 80 mol %. Theviscosity of the polyvinyl acetals (DIN 53015; Höppler method, 10%strength solution in ethanol) is at least 2 mPas to 1200 mPas,preferably 4 to 60 mPas. The degree of polymerization is at least 100.

When compared with the processes described in the prior art, theinventive process for preparing polyvinyl acetals is not only moreenvironmentally compatible, because there is less pollution ofwastewater, but also more cost-effective, because a major portion of theraw materials, in particular of the acid, can be saved.

The inventive process for preparing polyvinyl acetals, in particularpolyvinyl butyrals or mixed polyvinyl acetals, gives products which canbe used in printing ink compositions.

Suitable printing ink formulations are known to the person skilled inthe art and generally comprise 5 to 30% by weight pigment content, forexample disazo pigments or phthalocyanine pigments, 5 to 20% by weightof polyvinyl acetal binder and solvent, for example alcohols, such asethanol, or esters, such as ethyl acetate. Where appropriate, otheradditives may also be present, for example retarders, plasticizers, andother added materials, such as fillers or waxes. Adhesion promoters mayalso be added, where appropriate.

The polyvinyl acetals accessible using the inventive process also havevery good suitability for laminated safety glass and glass composites,and high-performance safety glass, or glazing films.

Water-soluble, partially acetalized polyvinyl acetals prepared by theinventive process, in which ionic groups may also be present, forexample carboxylate groups or sulfonate groups, are moreover used as aprotective colloid, for example for aqueous dispersions and duringpolymerization in an aqueous medium, and during the preparation ofwater-redispersible dispersion powders. Preference is given here towater-soluble (solubility more than 10 g/l in water under standardconditions) polyvinyl acetals with a degree of acetalization of from 1to 20 mol %, in particular from 3 to 16 mol %.

The polyvinyl acetals prepared by the inventive process may also be usedin water-based coatings.

Other application sectors for the polyvinyl acetals prepared by theinventive process are use as binders in corrosion-protectioncompositions, as binders in the ceramics industry, specifically asbinders for ceramics prior to firing, binders for ceramic powders andmetal powders during injection molding (powder injection molding), andbinders for the internal coating of cans.

The examples below provide further illustration of the invention, but inno way restrict the same.

Comparative Examples 1 and 11 describe the known prior-art preparationof polyvinyl acetals without use of mother liquor. Examples 2-10 and12-20 reuse about 92% of the mother liquor from the preceding batches.

COMPARATIVE EXAMPLE 1

2617 ml of distilled water, 826 ml of 20% strength HCl, and 1355 ml of a20% strength aqueous solution of a polyvinyl alcohol (hydrolysis numberHN 20.0 mg KOH/g, viscosity 3.18 mPas (DIN 53015; Höppler method; 4%strength aqueous solution)) formed an initial charge in a 6 liter glassreactor. The initial charge was cooled to +5° C., with stirring, withina period of one hour. 100 ml of acetaldehyde, precooled to −4° C., wereadded within a period of 5 minutes. The internal temperature of thereactor rose here to +7° C. Within a very short time, the mixture wascooled again to +5° C. and stirred for 20 minutes at this temperature.128 ml of butyraldehyde, precooled to −4° C., were then added within thesame period of 5 minutes. The internal temperature rose here briefly to6.5° C. 5 to 7 minutes after addition of the butyraldehyde, theinitially clear batch became milky, and the product precipitated afteras little as 10 minutes. After 40 minutes of reaction time at +5° C.,the temperature was increased to 25° C. over a period of 3.5 hours, andmaintained for a further 1.5 hours. The product was then filtered offwith suction and washed with distilled water until the filtrate wasneutral. The mother liquor was stored for the next batch. The productwas then dried to a solids content of at least 98%, initially at 22° C.,then at 35° C. in vacuo.

This gave a polyvinyl acetal with 13.6% by weight of vinyl alcoholunits. The vinyl acetate content was below 2% by weight. The viscosity(DIN 53015; Höppler method; 10% strength ethanolic solution) was 19.0mPas.

INVENTIVE EXAMPLE 2

The procedure was based on Comparative Example 1, except that use wasmade of 3151 ml of mother liquor obtained during the course of thework-up from Comparative Example 1. The mother liquor was added to theinitial charge together with 1355 ml of the 20% strength aqueouspolyvinyl alcohol solution and 292 ml of 20% strength hydrochloric acid.This gave an acid concentration of 5.20% by weight, and the conductivityof the mixture was 410 mS/cm. The amount of acetaldehyde andbutyraldehyde, the precipitation, and the work-up were based onComparative Example 1.

This gave a polyvinyl acetal with 13.3% by weight of vinyl alcoholunits. The vinyl acetate content was below 2% by weight. The viscosity(DIN 53015; Höppler method; 10% strength ethanolic solution) was 19.0mPas.

INVENTIVE EXAMPLES 3-10

The procedure was based on Example 2, except that the aqueous mediumused comprised the proportion mentioned in Table 1 of mother liquor fromthe preceding batch. The acid concentration in the initial charge wasalways adjusted to 5.10-5.25% by weight.

The compositions of the batches are found in Table 1. The analysisresults for each of the final products are shown in Table 2.

COMPARATIVE EXAMPLE 11

2715 ml of distilled water, 1114 ml of 20% strength HCl, and 1172 ml ofa 20% strength aqueous solution of a polyvinyl alcohol (hydrolysisnumber HN 20.0 mg KOH/g, viscosity 3.18 mPas (DIN 53015; Höppler method;4% strength aqueous solution)) formed an initial charge in a 6 literglass reactor. The initial charge was cooled to −2° C., with stirring,within a period of one hour. 190 ml of butyraldehyde, precooled to −4°C., were then added within a period of 5 minutes. The internaltemperature of the reactor rose here to 0 C. Within a very short time,the mixture was cooled again to −2° C. About 3 minutes after addition ofthe butyraldehyde, the batch, which until now had been clear, becamemilky, and the product precipitated after as little as 5 minutes. After40 minutes of reaction time at −2° C., the temperature was increased to25° C. over a period of 3.5 hours, and maintained for a further 1.5hours. The product was then filtered off with suction and washed withdistilled water until the filtrate was neutral. The mother liquor wasstored for the next batch. The product was then dried to a solidscontent of at least 98%, initially at 22° C., then at 35° C. in vacuo.

This gave a polyvinyl butyral with 18.4% by weight of vinyl alcoholunits. The vinyl acetate content was below 2% by weight. The viscosity(DIN 53015; Höppler method; 10% strength ethanolic solution) was 18.5mPas.

INVENTIVE EXAMPLE 12

The procedure was based on Comparative Example 11, except that use wasmade of 3481 ml of mother liquor obtained during the course of thework-up from Comparative Example 11. The mother liquor was added to theinitial charge together with 1172 ml of the 20% strength aqueouspolyvinyl alcohol solution and 348 ml of 20% strength hydrochloric acid.This gave an acid concentration of 6.30% by weight, and the conductivityof the mixture was 477 mS/cm. The amount of butyraldehyde, theprecipitation, and the work-up were based on Comparative Example 11.

This gave a polyvinyl acetal with 17.5% by weight of vinyl alcoholunits. The vinyl acetate content was below 2% by weight. The viscosity(DIN 53015; Höppler method; 10% strength ethanolic solution) was 18.0mPas.

INVENTIVE EXAMPLES 13-20

The procedure was based on Inventive Example 12, but the aqueous mediumused in each case comprised the proportion mentioned in Table 3 ofmother liquor from the preceding batch. The acid concentration in theinitial charge was always adjusted to 6.20-6.31% by weight.

The compositions of the batches are found in Table 3. The analysisresults for each of the final products are shown in Table 4.

Comparison of Comparative Example 1 (standard preparation method formixed polyvinyl acetals) with Inventive Examples 2-10 shows that thereis no change in the analytical or application-related product propertiesas a result of the (repeated) use of the mother liquor from precedingbatches.

Comparison of Comparative Example 11 (standard preparation method forpolyvinyl butyrals) with Inventive Examples 12 to 20 shows that there isno change in the analytical or application-related product properties asa result of (repeated) use of the mother liquor from preceding batches.

Determination Methods

1. Determination of the Dynamic Viscosity of a Solution of PolyvinylAcetals (Solution Viscosity):

90.00±0.01 g of ethanol and 10.00±0.01 g of polyvinyl acetal wereweighed out in a 250 ml Erlenmeyer flask with ground glass stopper andcomplete solution was achieved at 50° C. in a shaker. The mixture wasthen cooled to 20° C., and the dynamic viscosity (DIN 53 015; Höpplermethod) at 20° C. was determined using a suitable ball, e.g. ball 3.

2. Determination of Vinyl Alcohol Content:

The content of vinyl alcohol groups in the polyvinyl acetals wasdetermined by acetylating the hydroxy groups with acetic anhydride inthe presence of pyridine and 4-dimethylaminopyridine.

To this end, 1 g±0.001 g of polyvinyl acetal was dissolved in 24 ml ofpyridine and 0.04 g of 4-dimethylaminopyridine at 50° C. within a periodof 2 hours. The solution, cooled to 25° C., was treated with 10 ml of amixture composed of pyridine and acetic anhydride (87/13 parts byvolume) and intimately mixed for one hour. 30 ml of a mixture composedof pyridine/water (5/1 parts by volume) were then added and the mixturewas shaken for a further hour. Methanolic 0.5 N KOH was then used fortitration to pH 7.

Calculation

% by weight of vinyl alcohol=[(100×M_(w))/2000]×(ml blind value−mlspecimen), where M_(w)=average molecular weight per repeat unit of thepolymer.

3. Determination of the Viscosity of the Polyvinyl Alcohol Solutions:

The viscosity of the partially or fully hydrolyzed solid resins used asstarting materials was determined by a method based on the determinationof the dynamic viscosity of the polyvinyl acetals; only 4% strengthaqueous solutions were used.

TABLE 1 Amount of water (mother liquor) + HCl = 3443 ml; PVAL 20%strength: 1335 ml Mother Initial charge Precursor mother liquorliquor/HCl data HCl Mother 20% HCl conc. Conductivity liquor strengthconc. Conductivity Example Precursor in % [mS/cm] [ml] HCl [ml] in %[mS/cm] c1 — — — water 826 5.1 417 2617 2 c1 3.72 307 3151 292 5.20 4103 2 3.81 307 3169 274 5.25 401 4 3 3.82 307 3171 272 5.20 397 5 4 3.88305 3182 261 5.18 391 6 5 3.85 300 3177 266 5.14 388 7 6 3.82 294 3171272 5.15 387 8 7 3.82 294 3171 272 5.18 389 9 8 3.86 299 3178 265 5.23388 10  9 3.89 297 3184 259 5.19 386 3.90 296

TABLE 2 Solids content of Mother liquor GC Final product GC motherAcetaldehyde Butyraldehyde Acetaldehyde Butyraldehyde liquor Example[ppm] [ppm] [ppm] [ppm] [%] c1 1800 8980 9.5 575 — 2 3600 10300 13 5150.01 3 4380 11300 20 520 0.30 4 4720 11100 25 1020 0.28 5 5550 11900 13790 0.25 6 5500 11900 20 1300 0.26 7 5600 12100 33 2150 0.33 8 605011800 27 1200 0.42 9 5650 11800 41 1550 0.33 10  5380 13000 22 1200 0.250.37 Analyses of final products OH Moisture Conductivity numberViscosity EtAc Example Appearance [%] pH [μS/cm] [%] [mpas] solubilityc1 white 0.6 5.2 15.0 13.6 19.0 clear 2 white 0.8 5.1 15.0 13.3 19.0clear 3 white 1.0 4.8 18.3 13.5 19.8 clear 4 white 0.8 4.8 10.9 13.419.4 clear 5 white 0.8 4.6 21.4 13.2 19.2 clear 6 white 0.6 4.4 26.113.2 19.3 clear 7 white 1.2 4.4 28.3 13.3 18.9 clear 8 white 1.0 4.422.6 13.1 19.0 clear 9 white 1.3 4.4 24.9 13.1 19.7 clear 10  white 1.34.3 23.3 13.2 19.4 clear

TABLE 3 Amount of water (or mother liquor) + HCl = 3829 ml; PVAL 20%strength: 1172 ml Mother liquor/HCl Initial charge Mother liquorprecursor 20% data HCl Conductivity Mother strength HCl ConductivityExample Precursor conc. % [mS/cm] liquor HCl conc. % [mS/cm] c11 — — —water 1114 6.20 482 2715 12 c11 4.82 386 3481 348 6.30 477 13 12 4.96388 3513 316 6.25 472 14 13 4.91 388 3502 327 6.20 470 15 14 4.92 3873504 325 6.25 469 16 15 4.90 382 3499 330 6.30 470 17 16 4.93 381 3506323 6.31 468 18 17 4.92 380 3504 325 6.30 469 19 18 4.96 386 3513 3166.30 467 20 19 4.97 380 3516 313 6.21 462 4.99 380

TABLE 4 Solids Mother liquor GC Product GC content of ButyraldehydeButyraldehyde mother liquor Example [ppm] [ppm] [%] c11 3370 230 — 124250 240 0.02 13 5120 320 0.20 14 4420 380 0.14 15 5050 415 0.22 16 5100365 0.20 17 5150 380 0.26 18 4700 500 0.32 19 4760 410 0.29 20 5020 3000.35 0.33 Analyses of final products 10% strength OH EtOH MoistureConductivity number viscosity EtAc Example Appearance [%] pH [μS/cm] [%][mPas] solubility c11 white 0.7 5.0 10.8 18.4 18.5 clear 12 white 0.94.8 13.4 17.5 18.0 clear 13 white 0.6 4.5 18.6 16.8 18.5 clear 14 white0.8 4.6 12.1 16.9 18.1 clear 15 white 0.7 4.5 16.1 17.1 18.0 clear 16white 0.9 4.6 15.2 16.3 18.0 clear 17 white 0.9 4.6 18.0 16.4 17.9 clear18 white 0.8 4.4 18.5 16.4 18.0 clear 19 white 1.0 4.2 24.3 16.4 18.0clear 20 white 1.0 4.3 23.9 16.5 18.0 clear

1. A process for preparing polyvinyl acetals by hydrolysis of a vinylester polymer and acetalization of the resultant polyvinyl alcohol withone or more aldehydes selected from the group consisting of aliphaticand aromatic aldehydes having 1 to 15 carbon atoms, comprising: a)providing a polyvinyl alcohol to form an initial charge in an aqueousmedium, b) if necessary, adding acid to adjust to a pH of <1, c)initiating acetalization by addition of an aldehyde component, and d)precipitating polyvinyl acetal and separating polyvinyl acetal viafiltration from a filtrate, wherein from 10 to 100% by weight of theaqueous medium of a) comprises the filtrate from a preceding batch ofpolyvinyl acetal production, and, optionally reusing the filtrateproduced in d) in a further polyvinyl acetal production batch.
 2. Theprocess of claim 1, wherein the filtrate produced in step d) is reusedin a further production of polyvinyl acetal.
 3. The process of claim 1,wherein the vinyl ester polymer contains one or more monomer unitsselected from the group consisting of vinyl acetate, vinyl propionate,vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinylacetate, vinyl pivalate, and vinyl esters of α-branched monocarboxylicacids having 5 to 11 carbon atoms.
 4. The process of claim 2, whereinthe vinyl ester polymer contains one or more monomer units selected fromthe group consisting of vinyl acetate, vinyl propionate, vinyl butyrate,vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinylpivalate, and vinyl esters of α-branched monocarboxylic acids having 5to 11 carbon atoms.
 5. The process of claim 1, wherein the vinyl esterpolymer used comprises polyvinyl acetate, a vinyl acetate-1-methylvinylacetate copolymer, or a vinyl acetate (meth)acrylic acid copolymer. 6.The process of claim 2, wherein the vinyl ester polymer used comprisespolyvinyl acetate, a vinyl acetate-1-methylvinyl acetate copolymer, or avinyl acetate (meth)acrylic acid copolymer.
 7. The process of claim 1,wherein the polyvinyl alcohol has a degree of hydrolysis of 50 to 100mol %.
 8. The process of claim 1, wherein the aldehyde(s) comprise oneor more selected from the group consisting of formaldehyde,acetaldehyde, propionaldehyde, butyraldehyde, and benzaldehyde.
 9. Theprocess of claim 1, wherein the polyvinyl alcohol forms an initialcharge in the form of an aqueous solution with a concentration of 10 to50% by weight solids, and the solids content is altered by addition ofthe filtrate from a preceding polyvinylacetal acetalization.
 10. Theprocess of claim 1, wherein polyvinyl acetals are obtained with a degreeof acetalization of 45 to 80 mol % and with a degree of polymerizationof at least 100.